The present invention generally relates to optical disks, optical disk units and optical disk producing methods, and more particularly to an optical disk which has land regions for writing information arranged intermittently in a radial direction of the optical disk and also writes information in a region between the land regions. The present invention also relates to an optical disk unit which writes and read information to and from such an optical disk, and to an optical disk producing method for producing such an optical disk.
Generally, optical disks can roughly be divided into three kinds, namely, the so-called ROM optical disk, write once optical disks, and rewritable optical disks. The ROM optical disk is typified by a compact disk (CD) and the CD-ROM. The write once optical disk is primarily used in filing systems which process image information, and can write information only once but the written information can be read many times. On the other hand, the rewritable optical disk can treat code information, and the information can be written, erased and read many times.
However, the so-called partial ROM (P-ROM) optical disk has also been proposed. This P-ROM optical disk has a region which is exclusively for reading and is written with information such as a program, and a region which is used to write and read information such as data which require rewriting. Hence, the P-ROM optical disk has the functions of both the ROM optical disk and the rewritable optical disk. In such a P-ROM optical disk, there are demands to write information with a high density and to accurately read the written information, so as to further improve the performance and capacity of the optical disk.
FIG. 1 shows an example of a conventional optical disk, where (A) shows a perspective view of the optical disk and (B) shows an encircled portion of the optical disk in (A) on an enlarged scale.
A P-ROM optical disk 1 shown in FIG. 1 (A) has a region (hereinafter referred to as a ROM region) 2 exclusively for reading, and a region (hereinafter referred to as a write region) 3 which is used to write and read information. The write region 3 includes a region 3a to which the user can arbitrarily write information and from which the user can arbitrarily read the written information, and a preformat region 3b which is provided at periodic positions. This preformat region 3b is preformed during the disk production stage in the form of pits, and indicates an identification (ID) signal such as the track number and the sector number.
A recording layer 6 is formed on a substrate 5 as shown in FIG. 1 (B). A head guide groove 7 for tracking a light spot of an optical head (not shown) to scan a predetermined track is also formed on the optical disk 1. The head guide groove 7 may be made up of a single spiral groove or concentric grooves. In FIG. 1 (B), R indicates the disk rotating direction.
A land part 8 is formed between two head guide grooves 7 which are mutually adjacent in the radial direction of the optical disk 1. A pit sequence made up of intermittent pits 9 are preformed in the land part 8 of the ROM region 2 during the disk production stage. This pit sequence relates to information which does not need to be rewritten, such as programs, images and character fonts. After the disk production stage, arbitrary information is written on the recording layer 6 of the write region 3 by the user as an arrangement of magnetization directions, and not in the form of pits.
FIG. 2 shows another example of a conventional optical disk, where (A) shows a perspective view of the optical disk and (B) shows a part of the optical disk in (A) on an enlarged scale.
An optical disk 10 shown in FIG. 2 (A) has a write surface 11 with a write region 11a. A spiral track or concentric tracks are preformed on the write region 11a, and a region which may be used for writing and reading is provided between two mutually adjacent preformed tracks. This optical disk 10 is proposed in a Japanese Laid-Open Patent Application No. 61-178752, for example.
As shown in FIG. 2 (B), the write region 11a includes a preformed track which is made up of intermittent pits 12 and is written with ROM (read-only) information exclusively for reading, and a flat writable region which is formed on the recording layer 6 between two mutually adjacent preformed tracks. The preformed track is not limited to the intermittent pits 12. For example, a Japanese Laid-Open Patent Application No. 61-68742 proposes a preformed track which is formed by a transition from amorphous to crystal state.
The optical disk 1 shown in FIG. 1 has the ROM region 2 provided in a part of the write area of the optical disk 1. For this reason, the write region 3 is reduced by an amount corresponding to the ROM region 2. On the other hand, the optical disk 10 shown in FIG. 2 can provide a larger write region compared to the optical disk 1, because the intermittent pits 12 which describe the ROM information is used to obtain a tracking error signal when writing information to and reading information from the recording layer 6.
But the intervals of the pits 12 change at random depending in the ROM information, and thus, the amplitude of the tracking error signal which is obtained from the pits 12 changes at random depending on the information content. For example, if a signal obtained by (2, 7) modulation of data is written as sequence of intermittent pits 12, the amplitude of the tracking error signal which is obtained from the pits 12 decreases as the interval of the pits 12 increases. As a result, the amplitude of the tracking error signal for a minimum pit interval (mark interval) 1.5 .tau. of the signal which is obtained by the (2, 7) modulation and the amplitude for a maximum pit interval (mark interval) 4 .tau. differ by approximately 3.5 times as may be seen from FIG. 3, where .tau. indicates the pit interval which corresponds to the bit period of the data. Therefore, the tracking error signal cannot be stably obtained from the optical disk 10, and there is a problem in that an accurate tracking control cannot be made.
In addition, no ID signal is written on the optical disk 10. For this reason, there is a problem in that it is difficult to quickly search a desired position on the optical disk 10 when writing, erasing or reading the information.
Moreover, if the optical disk 10 is loaded into an existing optical disk unit which is designed to read information from the optical disk 1 by tracking the land part 8, the tracking will be made to the write region 11a of the optical disk 10 and it will be impossible to read the ROM information from the optical disk 10.
Next, a description will be given of the ID signal. In the optical disk 1 shown in FIG. 1, a sector mark (SM) and an ID (address) are written in each preformat region 3b which precedes the region 3a of the write region 3. FIG. 4 shows a part of the optical disk 1 in a vicinity of the preformat region 3b on an enlarged scale, and FIG. 5 shows a part of FIG. 4 on an enlarged scale. In FIGS. 4 and 5, those parts which are the same as those corresponding parts in FIG. 1 are designated by the same reference numerals, and a description thereof will be omitted.
As shown in FIGS. 4 and 5, the land part 8 between two mutually adjacent guide grooves 7 forms a track T, and the guide groove 7 does not form a track. For example, an interval a between two mutually adjacent guide grooves 7 is 1.6 .mu.m, a width b of the guide groove 7 is 0.6 .mu.m, and a depth c of the guide groove 7 is 60 nm in FIG. 5.
The data is written in the region 3a as a change in the direction of perpendicular magnetization, that is, by the magneto-optic recording. On the other hand, an SM pit sequence 15 is formed at the head of the preformat region 3b, and an ID pit sequence 16 is formed in the preformat region 3b following the SM pit sequence 15.
The SM pit sequence 15 is provided to indicate that the ID follows this SM pit sequence 15. For this reason, the SM pit sequence 15 has the same pattern form all of the tracks T, and the crosstalk of the SM pit sequences 15 between the mutually adjacent tracks T does not become a problem. On the other hand, the ID pit sequence 16 indicates the track number, and the pattern of the ID pit sequence 16 differs for each track T. Accordingly, the crosstalk of the ID pit sequences 16 between the mutually adjacent tracks T does become a problem.
Next, a description will be given of an optical disk unit for playing the optical disk 1, by referring to FIG. 6.
An optical disk unit 20 generally includes an optical head 21, a tracking servo circuit 35, a demodulator 41, an SM detector 43, an ID detector 44, and a controller 45 which are connected as shown in FIG. 6.
In the optical head 21, a laser beam which is emitted from a semiconductor laser 22 passes through a collimator lens 23 and a beam splitter 24 and is converged on the optical disk 1 by an objective lens 25 as a spot 26. The laser beam is reflected by the optical disk 1 and is directed towards a .lambda./2 plate 27. The reflected beam passing through the .lambda./2 plate 27 is split into two beams by a polarization beam splitter 28. One beam from the polarization beam splitter 28 is converged on a photodetector 30 via a lens 29, and the other beam is converged on a split photodetector 32 via a lens 31.
A difference between an output of the photodetector 30 and an output of the split photodetector 32 is obtained via a differential amplifier 33, and a magneto-optic signal 40 is output from the differential amplifier 33. This magneto-optic signal 40 is obtained by reading the information which is written as a change in the direction of perpendicular magnetization by use of the change in the rotation of the polarization plane. On the other hand, a sum of the output of the photodetector 30 and the output of the split photodetector 32 is obtained via an amplifier 34, and a reflectance signal 42 is output from the amplifier 34. This reflectance signal 42 is obtained by reading the information (SM and ID) which is written as pit sequences by use of the change in the reflectance.
On the other hand, a differential amplifier 36 which forms the tracking servo circuit 35 obtains a difference of outputs from each of light receiving parts of the split photodetector 32, and outputs a tracking error signal. This tracking error signal is supplied to a tracking actuator 37 so as to carry out the tracking control.
As described above, the land part 8 of the optical disk 1 forms the track T, and the relationship between the output level of the tracking control signal with respect to the amount of tracking error becomes as indicated by a curve I in FIG. 7.
The magneto-optic signal 40 described above is supplied to the demodulator 40 which outputs a read signal. On the other hand, the reflectance signal 42 described above is supplied to the SM detector 43 and the ID detector 44. An output of the ID detector 44 is supplied to the demodulator 41 and the controller 45.
Next, a description will be given of a case where the spot 26 relatively scans the preformat region 3b and then the region 3a in FIG. 4.
In this case, the reflectance signal 42 is first output from the amplifier 34, and the SM detector 43 detects the SM in response to this reflectance signal 42. Then, responsive to the output of the SM detector 43, the ID detector 44 is made active and detects the ID.
When the ID detector 44 detects the ID, the demodulator 41 is made active in response to the output of the ID detector 44. Hence, the demodulator 41 demodulates the magneto-optic signal 40 which is output from the differential amplifier 33 following to the reflectance signal 42, and outputs the read signal which is related to the data written in the region 3a. On the other hand, the output of the ID detector 44 is also supplied to the controller 45, and the controller 45 controls the entire operation of the optical disk unit 20.
As described above, only the land part 8 between the two mutually adjacent guide grooves 7 is used as the track T, and the storage capacity of the optical disk 1 cannot be further improved notably. Hence, it is conceivable to apply the concept proposed in a Japanese Laid-Open Patent Application No. 60-18832 to the optical disk 1 in order to further improve the storage capacity. More particularly, it is conceivable to utilize the guide groove portion as a track in addition to the land part.
FIG. 8 shows an essential part of a conceivable optical disk 1A which is based on the above application. In FIG. 8, a land part 52 forms a track T, and a guide groove 53 also forms a track T. The data is written in a data region 54, and the ID is written in an ID region 55 in the form of an ID pit sequence 56.
Compared to the conventional optical disk 1, the data storage capacity of the conceivable optical disk 1A is approximately doubled. In addition, the data is written in the data region 54 by the magneto-optic recording, and the crosstalk from the track T which is adjacent to the target track T which is to be read was approximately -32 dB in the data region 54, thereby introducing no problem from the practical point of view. The present inventors have found that errors increase considerably if the crosstalk becomes approximately -24 dB.
On the other hand, in the ID region 55, the crosstalk from the track T which is adjacent to the target track T which is to be read was approximately -13 dB and large. As a result, the ID of the intended track T could not be read correctly, and there was a problem in that errors were introduced in the read data.
An optical disk having pits formed in a guide groove has been proposed in a Japanese Laid-Open Patent Application No. 2-73549. The pits are formed in the guide groove to improve the recording capacity. However, the Japanese Laid-Open Patent Application No. 2-73549 does not contain any disclosure related to an ID signal. Furthermore, no specific relationship is disclosed between the depth of the guide groove and the depth of the pits. But if the depths are not optimized, the tracking error signal amplitude will fluctuate and will result in an inaccurate tracking control depending on the intervals of the pits, similarly to the case of the optical disk 10 having no guide groove. The necessity to optimize the depths of the guide groove and the pits will be apparent from the following description when read in conjunction with the accompanying drawings.